Many new materials being used nowadays in high technology sectors are sensitive to an atmosphere surrounding the material, i.e. to air and moisture for example. One example is materials which are used for organic electronic components, such as organic light-emitting diodes (OLEDs) for example.
Since the demonstration of low operating voltages by Tang et al. (cf. C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)), OLEDs have been promising candidates for producing new lighting and display elements. They comprise a sequence of thin layers of organic materials, which are preferably applied in vacuo by vapour deposition or in their polymeric form by spin-on deposition. After electrical contacting by means of metal layers, they form a wide range of electronic or optoelectronic components, such as diodes, light-emitting diodes or photodiodes for example. The thin organic layers can also be used to produce transistors. These organic components compete with the established components based on inorganic layers.
In the case of organic light-emitting diodes, light is generated by the injection of charge carriers (namely electrons from one side and holes from the other side) from the contacts into adjoining organic layers as a result of an externally applied voltage, the subsequent formation of excitons (electron/hole pairs) in an active zone and the recombination of these excitons to produce light, and said light is emitted from the light-emitting diode.
The advantage of such organic-based components over conventional inorganic-based components, for example semiconductors such as silicon or gallium arsenide, lies in the fact that it is possible to produce elements with a very large surface area, that is to say large display elements such as screens or display panels. The organic starting materials are relatively inexpensive compared to inorganic materials. Moreover, due to their lower processing temperature compared to inorganic materials, these materials can be applied to flexible substrates, which opens up a large number of new applications in the field of display and lighting technology.
Document U.S. Pat. No. 5,093,698 describes an organic light-emitting diode of the pin type, which is an organic light-emitting diode with doped charge carrier transport layers. In particular, use is made of three organic layers which are located between two contacts formed as electrodes. In said document, n-doped and p-doped layers improve the injection of charge carriers and the transport of both holes and electrons into the correspondingly doped layer. The energy levels HOMO (“Highest Occupied Molecular Orbital”) and LUMO (“Lowest Unoccupied Molecular Orbital”) are preferably selected such that both types of charge carrier are “trapped” in the emission zone so as to ensure efficient recombination of electrons and holes. The restriction of the charge carriers to the emission zone is achieved by suitably selecting the ionization potentials/electron affinities for the emission layer and/or the charge carrier transport layer. The emission layer can also be doped with fluorescent or phosphorescent emitter dopants.
The component structure known from document U.S. Pat. No. 5,093,698 leads to greatly improved charge carrier injection from the contacts into the organic layers. The high conductivity of the doped layers moreover reduces the voltage drop which occurs there during operation of the OLED. For a desired light density, doped components may therefore require much lower operating voltages than comparable undoped structures.
Particularly the dopants for the electron transport layer are very sensitive to oxygen and moisture. Since these molecules are very strong donors in the case of n-doping, they react at least partially with the surrounding air even at room temperature, and thereafter cannot perform, or at least cannot fully perform, their function in the OLED. This applies both to inorganic n-dopants such as caesium and to organic n-dopants such as decamethylcobaltocene.
It is known that emitter dopants, too, react with the oxygen in the air and with moisture at room temperature, as a result of which the efficiency of the OLED is reduced and possibly the spectrum of light emission is shifted. One example of such a reactive material is the phosphorescent emitter tris(1-phenylisoquinoline) iridium (III). However, other materials used in organic components may also react with the surrounding atmosphere at room temperature.
This means that such materials have to be protected from ambient air during the production of the organic components and also once the latter have been finished. Since such components are encapsulated after they have been produced, for example by means of a so-called thin layer encapsulation or by adhesive bonding to a glass or metal cap, the production of the component is particularly critical. During this production, the air-sensitive material is thermally evaporated in a vacuum installation and deposited on a substrate of the component. This method step is not critical for the air-sensitive material, since it usually takes place in vacuo and the air-sensitive material is therefore unable to react with ambient air. By contrast, the transport of the air-sensitive material to the vacuum coating chamber and the charging of the latter with the air-sensitive material are critical. Charging via an inert gas glove box is usually not possible, and therefore charging via ambient air is necessary.
The charging of a conventional coating installation with a vapour deposition material, which in particular is sensitive to air, for coating by means of evaporation should as far as possible take place without the vapour deposition material that is to be processed being exposed to the ambient air and without requiring modification of the coating installation. For this purpose, the vacuum chamber is usually connected to a glove box which is filled with inert gas, such as nitrogen or argon for example. The sensitive vapour deposition material is then charged into the chamber via the glove box. This procedure is not very practical since most coating installations are not connected to a glove box and the provision and maintenance of such a glove box are very costly and complicated.
As an alternative, attempts have been made firstly to fill the vapour deposition material into containers which are flooded with an inert gas, and then to bring the material into the vacuum chamber as quickly as possible and rapidly evacuate the latter. With this procedure, contact with the ambient air is admittedly reduced, but this charging procedure is not sufficient for many of the highly reactive vapour deposition materials. Even the brief contact of the sensitive material with the ambient air is sufficient for the material to at least partially react with the oxygen or water.
Another alternative consists in using a source for the vapour deposition material which can be separated from the evaporation installation, wherein the source has to be sealed in a gas-tight manner after it has been removed. The source can then be filled with the air-sensitive vapour deposition material in a glove box under inert gas, sealed in an air-tight manner, transferred out of the glove box and re-connected to the coating installation. As soon as there is no longer any oxygen or water in the vacuum installation, the separation between source and coating installation is lifted, and the vapour deposition process can begin without it being possible for the air-sensitive material to react. One disadvantage of this variant is that such sources are not very widespread. They are expensive and the charging procedure is complicated.
A method to deposit a vapour deposition material is known (see patent abstract of JP 59031865), wherein a vapour deposition batch of a vapour deposition material is placed in a housing, which is then closed with a lid. The lid is made of a material, which can be broken up by means of heating or internal pressure. By way of an example a thin film made of quartz can be used. Laser light is irradiated on the vapour deposition batch to evaporate the vapour deposition batch of the vapour deposition material during deposition.